Burners of the type to which the present invention relates usually have a gasifier to which fuel can be supplied, for example via an atomizing nozzle, an air inlet, and, at the outlet, a distribution device. The distribution device may have a plurality of openings therein.
German Patent 19 51 752, Brodlin, describes a burner having a mixture distribution body located spaced from a fuel nozzle. The mixture distribution body is intended to finely divide liquid fuel, which is not yet mixed with combustion air, at its surface, so that a larger surface is available for impingement by the combustion air than of the fuel itself. Additionally, the mixture distribution body is intended to form a stabilizer for the flame which will result. It is noted in the publication that prior mixture distribution bodies could not obtain complete gasification, so that the flame will burn with a blue flame color, that is, without smoking or formation of soot.
Blue flames can be obtained with recirculation burners; burners of such types, however, are very expensive and useful for central heating plants only under limited conditions since the combustion chambers of such plants vary widely and uniform operation of all burners could not be assured.
The mixture distribution body described in the referenced German Patent 19 51 754, Brodlin, was stated to have a diameter of 45 mm, and formed with openings or bores, spaced center-to-center by 12 mm, of clear openings of 8 mm. These openings were distributed over the entire surface of the body. The air stream, insofar as it does not impinge on the body structure, passes through these openings.
The openings were intended to conduct heat derived from the flame which occurs at the body to the forward part of the mixture distribution body on which the partially gasified oil-air mixture impinges. The material of the mixture distribution body, remaining between the bores, was of sufficient size to ensure a generally uniform heat distribution or, respectively, an essentially uniform heat flow.
It has been found in actual practice that the burner structure as proposed did not fulfill the expectations. As described in German Patent 28 33 686, Kopp, instabilities and deposits of coke arise upon starting and during warm-up of the burner. Such instabilities and coke deposits result in high noise levels in operation and, further, in noxious exhaust gases.
German Patent 25 53 953, Kopp, assigned to the same assignee as the first-mentioned German Patent 19 51 752, Brodlin, proposes a switch-over device which can be so changed that, during starting and warm-up operation, combustion air is made turbulent in the region of the atomizing nozzle. Upon starting and warming up, this burner, then, operates with a yellow flame. After the burner has warmed up, the switch-over device is operated, so that, after heating of the distribution body, and continuous operation, combustion air is supplied in essentially laminar flow, without turbulence.
It has been found that this solution has the disadvantage of increased technical requirements and controls, and the danger always arose that the transfer mechanism did not operate properly. The turbulent yellow flame is noisy, and coking problems still arose. Additionally, the burner cannot meet current clean-air requirements.
The burner of the German Patent 28 33 686, Kopp, uses a mixture distribution body in combination with a switch-over device. Combustion air is applied, during starting and the warm-up phase, in form of a tubular hollow jet to the mixture distribution body without turbulence, however. Upon switch-over, that is, in continuous operation, the combustion air is provided in form of a beamed, tightly cohesive or bundled jet to the interior region of the mixture distribution body. This burner, also, is subject to malfunction if the switch-over device does not operate properly.
Two types of mixture distribution bodies have been proposed. One such body is essentially hemispherical; this element operates, in continuous steady-state operation, approximately similarly to the body shown in the aforementioned German Patent 19 51 752, Brodlin, which, however, did not find commercial acceptance for continuous operation due to the high coking deposits formed in operation. In another embodiment, the mixture distribution body has a plurality of axially staggered frusto-conical rings. The inner diameter of subsequent rings--in flow direction of the air--is smaller, or equal to the outer diameter of the immediately preceding ring. At the forward end, a cover with preferably six openings is provided.
In continuous operation, a concentrated beam or jet of air is applied tangentially to the mixture distribution body to surround it, and to induce in the circular slits between the rings back-flow or back-streams of hot combustion gases which flow through the fuel which slips off the rings, for gasing the fuel. A comparatively small portion of the fuel which impinges on the mixture distribution body flows, together with combustion air, through the openings in the cover and into the interior of the mixture distribution body, so that small yellow flames will result. The proportion of combustion air there available is small, so that these small flames which burn yellow are smoky and cause soot. They are needed, however, since they stabilize the overall combustion. It is believed that the stabilizing effect is due to heating of the mixture distribution body so that it can effectively hold the flame.
It has been found, in operation, that combustion with this burner results in excessively high nitrogen-oxyide (NO.sub.x) emission; carbon monoxide emission also is high, and the overall exhaust gases do not meet clean-air requirements.
The stream of air which surrounds, in part, the mixture distribution body has the effect of sucking combustion gases out of the combustion chamber. They stream along the mixture distribution body and cause heating of its surface In dependence of the dimensioning of the combustion chamber, the combustion gases fed back are more or less hot, so that sufficient vaporization heat is not necessarily available in all cases. This type of recirculation does not cause intensive mixing with the fuel. Reliable operation of the burner, thus, is not ensured and had led to the comments in the aforementioned literature that the recirculation burner has disadvantages.
The burner in accordance with the German Patent 28 33 686, Kopp, generates a relatively high proportion of thermal NO.sub.x. Due to the Coanda effect (the wall attachment phenomenon of fluid jets), the stream of the air-fuel mixture follows along the outer wall of the mixture distribution body. This outer wall, at the end, is parallel to or at an acute angle with respect to its axis, so that the air leaves in essentially axial direction. This is a very hot flame which constricts towards its axis, a flame which inherently enhances the formation of NO.sub.x gases.
U.S. Pat. No. 4,624,631, Kobayashi et al, describes a kerosene burner in which a hollow conical or hemispherical burner cup of porous ceramic material is located within a porous ceramic burner chamber. This is a kerosene burner, and the problems which were discussed in connection with the German Patent 28 33 686, Kopp, arise similarly in this structure.
All the burners described heretofore have in common that liquid fuel, for example oil drops, impinge on a body. This body may be termed a mixture distribution body, a burner cup or the like. This body is heated by recirculation by the flames which arise at the holes in the body. In the burner of the German Patent 23 33 686, Kopp, the fuel drops impinge on the conical rings, and it is intended that the fuel which drops or slips off the rings is gasified by the recirculation of hot combustion gases. In all the burners described heretofore gasification and mixing of fuel with air are not clean, or clearly defined processes both with respect to time as well as with respect to location. The mixture of gasified fuel and air thus is not homogeneous. It has been found that after extended operation of the burner, the geometry of the stream emitted from the nozzle will change, so that the spray cone emitted therefrom becomes irregular. Consequently, the mixture distribution body, or the combustion cup, respectively, will no longer be uniformly heated by the flames arising therebeyond. This non-uniformity, again, interferes with vaporization of the fuel, with the result that the generation of carbon monoxide increases; unburned hydrocarbon components have even been found in the exhaust gases. An additional factor is an increase in noise level in operation of the burner.
The yellow, smoking flames which arise within the cavity of the mixture distribution body, or in a combustion cup, are necessary in order to provide the necessary heat for vaporization of the fuel. Sometimes these flames may be blue. These flames generate very high temperatures within the cavity which, again, leads to excessive production of NO.sub.x gases in operation of such burners.
The referenced U.S. patents by the inventors hereof describe a recirculation burner in which, downstream from the fuel nozzle, a gasification space is first located followed by a mixing head, and then a deflection arrangement. In contrast to the previously described burners with mixture distribution bodies, which do not effect a continuous change in direction of the flame, the burners of the referenced patents by the inventors hereof are constructed to provide for deflection of the flame in essentially radial direction. Thus, and in contrast to the arrangement of the German Patent 19 51 752, Brodlin, the burners of the referenced U.S. Pat. No. 4,957,427, Fullemann et al, U.S. Pat. No. 5,015,173, Fullemann et al, U.S. Pat. No. 5,154,597, Fullemann et al, cause vaporization of the fuel and mixing of the vaporized fuel with air in separate steps. The German Patent 19 51 752, Brodlin, was intended to replace the previously expensive recirculation burners with the simple mixture distribution element. In accordance with the referenced patents by the inventors hereof, vaporization of the fuel is effected first by a gasifier which is heated by hot recirculation gases. Thereafter, the now gasified fuel is mixed with air. This mixture then can leave the mixing head by a plurality of slit-formed exit openings. Gasifier and mixing head are surrounded by a flame tube which extends about to the end of the deflection arrangement, and which also causes formation of a recirculation path to the gasifier space. This deflection arrangement, in contrast to the prior art, does not function as the gasifier itself; it is not heated, and, looking at it first, one cannot see why or if it should be heated.